Electroplating method and apparatus

ABSTRACT

The interior surfaces of each of a plurality of hollow metal objects are simultaneously electroplated with a uniform layer of electrodeposited metal by positioning an anode centrally within each of the hollow objects and electrically connecting the anodes and the objects being electroplated in series with each other and with a source of an electrolyzing current. An electrically conductive electrolyte solution is continuously supplied to the interior of each of the hollow objects and an equivalent quantity of the solution is continuously withdrawn from each of the objects and returned to the source of supply of the solution. The electrolyte solution is introduced into and is withdrawn from each object through a conduit that provides a path of high electrical resistance to the flow of an electric current through the electrolyte contained in said conduit so that the stray electrolyzing currents flowing from any of the objects through the solution in the conduit are reduced to a value that will not significantly affect the electroplating operation taking place in the other objects being electroplated.

United States Patent Raymond et al.

[ 1 ELECTROPLATING METHOD AND APPARATUS Inventors: Louis W. Raymond;Roger E. Reath,

c/o Superior Plating Company, 2500 Post Rd., both of Fairficld, Conn.06430 Filed: July 12, 1974 Appl. No.: 488,065

Primary E.\aminerT. M. Tufariello Attorney, Agent, or Firm-Pennie &Edmonds I20 24 12b 24 12c 5 7 ABSTRACT The interior surfaces of each ofa plurality of hollow metal objects are simultaneously electroplatedwith a uniform layer of electrodeposited metal by positioning an anodecentrally within each of the hollow objects and electrically connectingthe anodes and the objects being electroplated in series with each otherand with a source of an electrolyzing current. An electricallyconductive electrolyte solution is continuously supplied to the interiorof each of the hollow objects and an equivalent quantity of the solutionis continuously withdrawn from each of the objects and returned to thesource of supply of the solution. The electrolyte solution is introducedinto and is withdrawn from each object through a conduit that provides apath of high electrical resistance to the flow of an electric currentthrough the electrolyte contained in said conduit so that the strayelectrolyzing currents flowing from any of the objects through thesolution in the conduit are reduced to a value that will notsignificantly affect the electroplating operation taking place in theother objects being electroplated.

4 Claims, 4 Drawing Figures 24 I2d 24 S w 1 lc lld t *9 pp y TankELECTROPLATING METHOD AND APPARATUS BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to the simultaneouselectroplating of the interior surfaces of a plurality of hollow metalobjects.

2. Prior Art It is frequently desirable to provide the interior surfaceof a hollow metal object with a corrosion resistant and/or wearresistant layer of electrodeposited metal. For example, gun barrels andthe cylinder walls of internal combustion engines are frequentlychromium plated to provide hard, wear-resistant interior surfaces forthese objects. In one method for accomplishing this result, the exteriorand all other surfaces of the objects except the interior surfaces to beelectroplated, are masked or coated with a non-conductive material sothat the object can be totally immersed in an electrolyte solutioncontained in a conventinal electrolyzing tank and the interior surfacesthereof electroplated in the conventional manner. In a preferredprocedure to which the present invention pertains, the hollow metalobject is employed as the container itself for the electrolyte solution,the openings of the hollow object being sealed off and electrolytesolution being supplied to and being withdrawn from the interior of theobject through suitable conduits connected to a source of supply of theelectrolyte solution. An anode is positioned centrally within the hollowobject, and an electrolyzing current is passed from the anode throughthe electrolyte solution within the object to the interior metal surfacebeing electroplated. In this procedure, the hollow metal object, theanode and the electrolyte solution contained in the object comprise, ineffect, a selfcontained electroplating cell.

If a number of hollow metal objects are to be electroplated it isdesirable from the standpoint of both economy and efficienty that all ofthe objects, or as many of the objects as possible, be electroplated atthe same time. Moreover, it is frequently very important that the layerof electrodeposited metal on the interior surfaces of all of the objectsbeing electroplated have the same uniform and reproducible thickness.When a number of hollow metal objects are to be electroplated inaccordance with the procedure to which the present invention pertains,it has theretofore been the general practice either to connect thecentrally positioned anode within each hollow metal object and thehollow metal object (the cathode) with which the anode is associateddirectly to their own individual source of electrolyzing current(usually a rectifier), or to connect all of the anodes and all of themetal objects (the cathodes) in parallel to a common source ofelectrolyzing current. The use of an individual rectifier for eachhollow metal object being electroplated is, in most cases, expensive. Onthe other hand, if all of the anodes and all of the cathodic metalobjects are connected in parallel to a sin gle rectifier, it isextremely difficult to control precisely the electrolyzing currentflowing through the electrolyte solution in each object and hence tocontrol the thickness of the layer of metal being electrodeposited onthe interior surfaces of each object.

In order to obtain an electrodeposited layer of uniform thickness on theinterior surfaces of each of the hollow metal objects it is necessarythat the same quantity of electrolyzing current be caused to flowthrough the electrolyte in each object. We have discovered that the mosteffective way to accomplish this result is to connect the anodes and thehollow objects being electroplated in series with each other and withthe source of electrolyzing current so that the same current(theoretically, at least) will flow through all of the objects in theseries. However, despite a very substantial improvement in theuniformity of the thickness of the layer of electrodeposited metal, wehave found that significant and troublesome variations remain in thethickness of the electrodepoited metal from one metal object to anotherin the series. Investigation of the problem has disclosed that strayelectrical currents flow through the electrically conductive electrolytesolution from one hollow object to the others in the series therebyadversely affecting the electroplating operation taking place in theother objects. After further investigation we have found that thesestray electrical currents can be reduced to an acceptably small value bycausing the electrolyte solution being supplied to and being withdrawnfrom each hollow object being electroplated to travel through arelatively long length of small diameter non-conductive plastic tubing.As a result, stray electric currents are reduced to an acceptableminimum, and uniform layers of electrodeposited metal are readilyobtained on the interior surfaces of seriesconnected hollow metalobjects.

SUMMARY OF THE INVENTION In our new procedure for simultaneouslyelectroplating the interior surfaces of each of a plurality of hollowmetal objects with a uniform layer of an electrodeposited metal, themetal objects are serially arranged with respect to each other and ananode is positioned centrally within each of the objects. The metalobjects and the anodes positioned within each object are thenelectrically connected in series with each other and with a source of anelectrolyzing current. An electrically conductive electrolyte solutionfrom a common source of supply of the solution is continuously suppliedto the interior of each of the hollow metal objects being electroplated,and an equivalent quantity of the solution is continuously withdrawnfrom each of the objects and returned to the common source of supply tothe solution. The electrolye solution is introduced into and iswithdrawn from each object through a conduit that provides a path ofhigh electrical resistance to the flow of an electric current throughthe electrolyte solution contained in said conduit so that strayelectrolyzing currents flowing from any of the objects through thesolution contained in the conduit is reduced to a value that will notsignificantly affect the electroplating operation taking place in theother objects being electroplated.

By way of example, each of the electrolyte solution supply and dischargeconduits comprises a separate length of non-conductive tubing. Each ofsaid lengths of tubing has a cross-sectional area and is of a lengthsuch that the resistance to the flow of an electrical current throughthe electrolyte solution contained in each length of tubing issufficiently great that the magnitude of stray electrolyzing currentflowing from any object in the series through the solution contained inthe tubing connected thereto is reduced to a value that will notsignificantly affect the electroplating operation taking place in otherobjects in the series.

BRIEF DESCRIPTION OF THE DRAWINGS The simultaneous electroplating of theinterior surfaces of a plurality of metal objects in accordance with theinvention will be better understood from the following descriptionthereof in conjunction with the accompanying drawings of which:

FIG. 1 is a schematic view of the overall arrangement of hollow metalobjects being electroplated connected in series with the source ofelectrolyzing current and in parallel with the source of electrolytesolution,

FIG. 2 is a schematic representation of the electrical characteristicsof the arrangement shown in FIG. 1,

FIG. 3 is a side view of two hollow metal objects the interior surfacesof which are being electroplated in accordance with the practice of theinvention, one of the metal objects being shown in section to illustratethe internal arrangement of the parts, and

FIG. 4 is a plan view of the apparatus shown in FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT As noted, the presentinvention relates to the simultaneous electroplating of the interiorsurfaces of each of a plurality of substantially identical hollow metalobjects with uniform layer of an electrodeposited metal. It isapplicable to the electroplating of any hollow metal object that can bemade to serve as the container for the electrolyte solution containingions of the metal being electrodeposited, including such objects as gunbarrels, reactor vessels, food processing equipment and utensils,machine parts and the like. The new procedure and the apparatus forcarrying out the procedure will be described in connection with thesimultaneous electrodeposition of a uniform layer of metallic chromiumon the cylindrical interior surfaces of removable metal liners for thecylinders of an internal combustion engine.

As shown schematically in FIG. 1 of the drawings, four substantiallyidentical cylinder liners 11a, 1 lb, 1 1c and 11d are arranged seriallywith respect to each other, and nonconsumable anodes 12a, 12b, 12c, and12d are positioned centrally within each cylinder liner. Power supplymeans 13 (advantageously, a rectifier R) are provivded for supplying anelectrolyzing current to the system, and current control means 14 andcurrent measurement means 15 are also provided for controlling andmeasuring the amount of electrolyzing current being supplied by theelectrolyzing current supply means. Electrical conductor meanselectrically connect the cylinder liners and the anodes disposed thereinin series with each other and with the current supply means so that thepositive pole of the source of electrolyzing current is electricallyconnected to the anode disposed within the first cylinder liner in theseries, each cylinder liner except the last liner is electricallyconnected to the anode disposed within the next succeeding cylinderliner in the series, and the last cylinder liner is electricallyconnected to the negative pole of the source of the electrolyzingcurrent. That is to say, with reference to FIG. 1 of the drawings, thepositive pole of the rectifier R is electrically connected to the anode12a centrally disposed within the first cylinder liner 11a, the firstcylinder liner 11a is electrically connected to the anode 12b centrallydisposed within the second cylinder liner I 1b, the second cylinderliner 11b is electrically connected to the anode 12c centrally disposedwithin the third cylinder liner llc, the third cylinder liner iselectrically connected to the anode 12d centrally disposed within thelast cylinder line 11d, and the last cylinder liner 11d is electricallyconnected to the negative pole of the rectifier.

A supply of electrolyte solution S (for example, an aqueous solution ofchromic acid) is contained in a supply tank 17. A pump 18 delivers theelectrolyte solution to the solution supply main 19 from whence it isdelivered to the interior of each of the cylinder liners, and excess oroverflow electrolyte solution is discharged from each cylinder liner andis returned to the supply tank 17 through the solution discharge main20. The electrolyte solution is supplied to each cylinder liner 11a, 1lb, 11c and 11d through individual solution supply conduits 21 each ofwhich communicates at one end with the solution supply main,19 and atthe other end with a solution inlet fitting 22 secured to the lower endof each cylinder liner. Similarly, the electrolyte solution isdischarged from each cylinder liner through individual solutiondischarge conduits 23 each of which communicates at one end with thesolution discharge main 20 and at its other end with a solutiondischarge fitting 24 secured to the upper end of each cylinder liner.

When electrolyte solution is being supplied to each of the cylinderliners and when an electrolyzing current is passed through thearrangement of series-connected anodes and cylinder liners, the sameamount of current will pass through the electrolyte solution containedin each of the cylinder liners and, theoretically at least, preciselythe same amount of metallic chromium will be electrodeposited on theinterior surfaces of each of the cylinder liners. As previouslymentioned, however, it has been found that stray electrolyzing currentsflow through the electrolyte solution from one cylinder liner to anotherin the series and, as a result, adversely affect the desired uniformityin thickness of the layer of electrodeposited metal in adjoiningcylinder liners.

We have found that these troublesome stray currents can be reduced to anacceptable level, if not entirely eliminated, by increasingsubstantially the electrical resistance to the flow of these currentsthrough the electrolyte solution being supplied to and being dischargedfrom each cylinder liner. That is to say, if each of the conduitsthrough which the electrolyte solution is supplied to and withdrawn fromeach cylinder liner provides a path of high electrical resistance to theflow of an electrical current through the electrolyte solution containedin the conduit, stray electrolyzing currents flowing from any of thecylinder liners through the solution in the conduit is reduced to avalue that will not significantly affect the electroplating operationtaking place in the other cylinder liners being electroplated.

In the presently preferred embodiment of the process the requiredincrease in the electrical resistance of the electrolyte solution isobtained by causing the solution being supplied to and withdrawn fromthe cylinder liners to travel through relatively long lengths ofrelatively small diameter non-conductive plastic tubing. That is to say,the resistance to the flow of an electric current through an electrolytesolution contained in a length of non-conductive plastic tubing variesdirectly with the length of the tubing and inversely with the square ofthe diameter of the tubing.,More specifically, the electrical resistanceof an electrolyte solution contained in a given length of plastic tubing6mm (A inch) in diameter is four times as great as the electricalresistance of the same solution contained in the same length of plastictubing 12mm (/2 inch) in diameter, and if the 6mm diameter tubing istwice as long the 12mm diameter tubing the electrical resistance of theelectrolyte solution contained in the smaller tubing will be eight timesthat of the solution contained in the larger tubing. Accordingly, in theembodiment shown in FIG. 1, the solution supply conduits 21 and solutiondischarge conduits 23 comprise lengths of non-conductive tubing ofsufficiently small diameter and of sufficiently great length to reducethe magnitude of stray electrolyzing current flowing therethrough fromone of the cylinder liners to a value that will not significantly affectthe electroplating operation taking place in the other cylinder linersin the series.

Other means may be employed to provide the necessary path of highelectrical resistance to the flow of an electric current through theelectrolyte solution contained in the solution supply and dischargeconduits. For example, each conduit may be fitted with two or morevalve-like elements which open and close alternately in succession toprovide a discontinuous path for the flow of an electric current. Otherequivalent measures may also be employed. In the preferred embodimentthe lengths of tubing employed as solution supply and discharge conduitsare substantially longer than would normally be required to connect thesolution supply main 19 to the solution inlet fitting 22 or to connectthe solution discharge main 20 to the solution discharge fitting 24 ofeach of the cylinder liners being electroplated, and preferably at leasttwo or three times as long as actually needed to connect the mains tofittings. Similarly, the tubing employed is of substantially smallerdiameter than the tubing that would normally be employed for thispurpose, and preferably at least about one half the diameter of suchtubing. The tubing itself must be made of an non-conductive materialsuch as polyethylene. The excess length of the small diameter tubing maybe conveniently accomodated by wrapping the tubing helically about thesolution mains as shown in the drawing.

The effect of causing electrolyte solution to flow through therelatively long lengths of relatively small diameter tubing 21 and 23 onthe electrical characteristics of the electroplating system is shownschematically in FIG. 2 of the drawing. The resistance to the flow ofelectrolyzing current through the electrolyte solution disposed betweenthe anode 12a and the cathodic cylinder liner 11a (and between the anode12b and the cathodic liner 11b, etc.) is represented by the resistancesymbol 26, and the resistance to the flow of stray electrolyzing currentthrough the elongated supply conduits 21 and discharge conduits 23 arerepresented by the resistance symbols 27 and 28, respectively. In orderto reduce the stray currents flowing through the electrolyte solutioncontained in the conduits 21 and 23 to acceptable levels, the ohmicvalue of each of the resistance 27 and 28 must be substantially greater(at least several times greater) than that of the resistance 26.

The required ohmic values of the resistances 27 and 28 may be determinedby first determining the maximum variation in the thickness of the layerof electrodeposited metal that can be tolerated, by computing theamperage of the stray currents required to plate or deplate a layerofmetal equal in thickness to a layer of the prescribed tolerance, and bycomputing the minimum ohmic values required to prevent the amperage ofthestray currents from exceeding the maximum amperage permitted. Thelength of the electrolyte supply and discharge conduits required toprovide these minimum ohmic values'is, of course, a function of thespecific resistivity of the electrolyte solution being used and may bedetermined by routine calculation. Alternatively, the selection oftubing of sufficiently small diameter and of sufficiently great lengthto meet this requirement may be accomplished by empirical observation.If the layer of electrodeposited metal is of uniform thickness in all ofthe hollow objects being electroplated, stray currents have been reducedto an acceptable level and the tubing is of the correct size and length.If the layer is not uniform and stray currents are the reason, a smallerdiameter tubing or a longer length of tubing must be employed.

In the embodiment shown in FIGS. 3 and 4 of the drawings, the cylinderliners 11a and 11b are serially disposed on the support structure 30.The solution inlet fitting 22 secured to the lower end of each cylinderliner comprises an outer cap member 31 and an inner seal member 32, andthe solution discharge fitting 24 secured to the upper end of eachcylinder lining comprises an outer cap member 33 and an inner sealmember 34. The inner seal members 32 and 34 provide a fluid tight sealbetween the lower and upper ends of the cylinder liner and the outer capmembers 31 and 33. The inner seal member 32 also serves as a solutiondistribution plate for the electrolyte solution flowing upwardly fromthe solution inlet chamber 35 into the interior of the cylinder lining.The inner seal members 32 and 34 are formed of a non-conductive plasticmaterial that is inert with respect to the electrolyte solution, forexample, polypropylene, and the outer cap members 31 and 33 areadvantageously, but not necessarily, formed of a similar material.

Non-consumable anodes 12a and 12b are centrally disposed within each ofthe cylinder liners 1 1a and 11b, each anode extending downwardlythrough appropriate openings formed in the outer cap member 33 and theinner seal member 34, respectively. The anode 12a is connected by aheavy gauge conductor 37 to the positive pole of the source ofelectrolyzing current (not shown), the cylinder liner 11a is connectedto the anode 12b by the heavy gauge conductor 38, and the cylinder liner11b is connected to the next anode in the series or to the negative poleof the source of electrolyzing current by the heavy gauge conductor 39.The conductors 38 and 39 are secured to their respective cylinder liners11a and 11b by means of metal clamp members 40 which insure a goodelectrical connection between the cylinder liners and the conductors.

An electrolyte solution supply main 19 and a solution discharge main 20are connected to a solution supply tank (not shown). Electrolytesolution from the solution supply main 19 is delivered to the solutioninlet fitting 22 secured to the lower end of each cylinder liner througha relatively long length of relatively small diameter plastic tubing 21,and overflow electrolyte solution from the solution discharge fittings24 secured to the upper end of each cylinder liner is delivered to thesolution discharge main 20 through relatively long lengths of relativelysmall diameter plastic tubing 23, all in accordance with the practice ofthe invention.

The following example is illustrative but not limitative of the practiceof the invention.

Ten steel liners for the cylinders of an internal combustion engine areprovided with solution inlet and solution discharge fittings and arearranged serially on a support bench, at non-consumable anode made oflead is disposed centrally within each cylinder liner, and the tenanodes and cylinder liners are electrically connected together in serieswith each other and with a source (a rectifier of 1000 amperes capacity)of electrolyzing current, all as shown in FIGS. 1 and 3 of the drawings.An electrolyte solution containing 32 ounces per gallon chromic acid(CrO is contained in a 500 gallon supply tank, the supply tank isconnected to solution supply and solution discharge mains made of 1 /2inch diameter plastic pipe, and the solution supply and discharge mainsare connected respectively to the solution inlet fittings and thesolution discharge fittings of the cylinder liners by means of solutionsupply and solution discharge conduits made of non-conductive plastictubing, as also shown in the drawings. Electrolyte solution from thesupply tank is circulated through all of the cylinder liners, the flowof the solution through each cylinder liner being controlled so that thesolution flows over the inner surface of each liner at the rate of 133ft/minute. An electrolyzing current is passed through the ten seriesconnected anodes and cathodic cylinder liners, the current being 500amperes at 50 volts (representing a voltage drop of 5 volts per liner).The electroplating operation is carried out in two separate periods orruns as described below.

In the first electroplating run each of the solution supply conduits andsolution discharge conduits comprises polypropylene tubing of 1 inchinside diameter and 5 feet in length. At the end of the electroplatingrun the layers of electrodeposited chromium metal on the interiorsurface of the cylinder liners are observed to be slightly non-uniformin thickness.

In the second electroplating run each of the solution supply conduitsand solution discharge conduits comprises polyethylene tubing of V2 inchinside diameter and feet in length. On completion of the electroplatingrun the layers of electrodeposited chromium metal on the interiorsurfaces of the cylinder liners are observed to be of substantiallyuniform thickness and quality.

We claim:

1. Method for simultaneously electroplating the interiors of each of aplurality of hollow metal which comprises,

positioning an anode centrally within each of the hol low objects beingelectroplated, electrically connecting said anodes and the objects beingelectroplated in series with each other and with a source of anelectrolyzing current,

continuously supplying an electrically conductive electrolyte solutionfrom a common source of supply of said solution to the interior of eachof the hollow objects being electroplated and continuously withdrawingan equivalent quantity of said selectrolyte solution from each of saidobjects and returning said solution to said common source of pp ypassing an electrolyzing current through the series of anode andelectrolyte-containing objects, and

causing the electrolyte solution being supplied to and being withdrawnfrom each object being electroplated to travel through solution inletand solution outlet passageways that provide a path of high electricalresistance to the flow of an electric current through the electrolytesolution contained in said passageways, whereby the magnitude of theelectrolyzing current flowing from any of the objects beingelectroplated through said electrolyte solution is reduced to a valuethat will not significantly affect the electroplating operation takingplace in other objects in the series.

2. The method according to claim 1 in which the solution inlet andsolution outlet passageways connected to the objects being electroplatedeach comprises a separate length of non-conductive tubing, each of saidlengths of tubing having a cross-sectional area and being of a lengthsuch that the resistance to the flow of an electrical current throughthe electrolyte solution contained in each length of tubing issufficiently great that the magnitude of stray electrolyzing currentflowing from any of said objects through the solution contained in saidtubing is reduced to a value that will not significantly affect theelectroplating operation taking place in other objects in the series.

3. The method according to claim 1 in which the required minimum ohmicvalue of the resistance to the flow of an electric current through theelectrolyte solution contained in each passageway is determined by firstdetermining the maximum variation in the thickness of the layer ofelectrodeposited metal that can be tolerated, by determining theamperage of stray currents required to plate or deplate a layer of metalequal in thickness to a layer of the prescribed tolerance, and bydetermining the minimum ohmic value required to prevent the amperage ofthe stray currents from exceeding the maximum amperage permitted.

4. Apparatus for simultaneously electroplating the interior surfaces ofeach of a plurality of hollow metal objects with a uniform layer of anelectrodeposited metal which comprises:

a plurality of serially disposed substantially identical hollow metalobjects to be electroplated, a nonconsumable anode centrally disposedwithin each of the hollow metal objects in the series, power supplymeans for supplying an electrolyzing current, and electrical conductormeans electrically connecting the hollow metal objects and the anodesdisposed therein in series with each other and with the electrolyzingcurrent power supply means,

a vessel adapted to contain a supply of electrolyte solution, anelectrolyte solution supply conduit connected to the electrolytesolution supply vessel and to each of the hollow metal obiects forsupplying electrolyte solution to the interior of each of said hollowmetal objects, and an electrolyte solution discharge conduit connectedto each of the hollow metal objects and to the electrolyte solutionsupply vessel for withdrawing electrolyte solution from each of saidobjects and returning said solution to said supply vessel,

the electrolyte solution supply conduit and discharge conduit connectedto each hollow metal object comprising a length of tubing ofelectrically insulating material, said tubing having a cross-sectionalarea and having sufficient length such as to cause the resistance to theflow of an electric current through the elctrolyte solution contained ineach of said lengths of tubing to be sufficiently great to reduce themagnitude of the electrolyzing current flowing therethrough from any ofthe objects being electroplated to a value that will not significantlyaffect the electroplating operation taking place in other objects in theseries.

1. MOTHOD FOR SIMULTANEOUSLY ELECTROPLATING THE INTERIORS OF EACH OF APLURALITY OF HOLLOW METAL WHICH COMPRISES, POSITIONING AN ANODECENTRALLY WITHIN EACH OF THE HOLLOW OBJECTS BEING ELECTROPLATED,ELECTROCALLY CONNECTING SAID ANODES AND THE OBJECTS BEINGS BEINGELECTROPLATED IN SERIES WITH EACH OTHER AND WITH A SOURCE OF ANELECTROLYZING CURRENT, CONTINUOUSLY SUPPLYING AN ELECTRICALLY CONDUCTIVEELECTROLYTE SOLUTION FORM A COMMON SOURCE OF SUPPLY OF SAID SOLUTION TOTHE INTERIOR OF EACH OF THE HOLLOW OBJECTS BEING ELECTROPLATED ANDCONTINUOUSLY WITHDRAWING AND EQUIVALENT QUANTITY OF SAID SELECTROLYTESOLUTION FROM EACH OF SAID OBJECTS AND RETURNING SAID SOLUTION TO SAIDCOMMON SOURCE OF SUPPLY, PASSING AN ELECTROLYZING CURRENT THROUGH THESERIES OF ANODE AND ELECTROLYTE-CONTAINING OBJECTS, AND
 2. The methodaccording to claim 1 in which the solution inlet and solution outletpassageways connected to the objects being electroplated each comprisesa separate length of non-conductive tubing, each of said lengths oftubing having a cross-sectional area and being of a length such that theresistance to the flow of an electrical current through the electrolytesolution contained in each length of tubing is sufficiently great thatthe magnitude of stray electrolyzing current flowing from any of saidobjects through the solution contained in said tubing is reduced to avalue that will not significantly affect the electroplating operationtaking place in other objects in the series.
 3. The method according toclaim 1 in which the required minimum ohmic value of the resistance tothe flow of an electric current through the electrolyte solutioncontained in each passageway is determined by first determining themaximum variation in the thickness of the layer of electrodepositedmetal that can be tolerated, by determining the amperage of straycurrents required to plate or deplate a layer of metal equal inthickness to a layer of the prescribed tolerance, and by determining theminimum ohmic value required to prevent the amperage of the straycurrents from exceeding tHe maximum amperage permitted.
 4. Apparatus forsimultaneously electroplating the interior surfaces of each of aplurality of hollow metal objects with a uniform layer of anelectrodeposited metal which comprises: a plurality of serially disposedsubstantially identical hollow metal objects to be electroplated, anon-consumable anode centrally disposed within each of the hollow metalobjects in the series, power supply means for supplying an electrolyzingcurrent, and electrical conductor means electrically connecting thehollow metal objects and the anodes disposed therein in series with eachother and with the electrolyzing current power supply means, a vesseladapted to contain a supply of electrolyte solution, an electrolytesolution supply conduit connected to the electrolyte solution supplyvessel and to each of the hollow metal objects for supplying electrolytesolution to the interior of each of said hollow metal objects, and anelectrolyte solution discharge conduit connected to each of the hollowmetal objects and to the electrolyte solution supply vessel forwithdrawing electrolyte solution from each of said objects and returningsaid solution to said supply vessel, the electrolyte solution supplyconduit and discharge conduit connected to each hollow metal objectcomprising a length of tubing of electrically insulating material, saidtubing having a cross-sectional area and having sufficient length suchas to cause the resistance to the flow of an electric current throughthe elctrolyte solution contained in each of said lengths of tubing tobe sufficiently great to reduce the magnitude of the electrolyzingcurrent flowing therethrough from any of the objects being electroplatedto a value that will not significantly affect the electroplatingoperation taking place in other objects in the series.